1. Field of the Invention
This invention relates to root attachment of composite blades to a rotor of gas turbine engines and, more particularly, to a low friction blade root to slot wall interface for composite blade composite roots.
2. Discussion of the Background Art
Gas turbine engine composite fan blades have dovetails or roots carried by a slot in a metal disk or drum rotor. During operation, under high compressive loads and relative movement between the root and a wall of the slot (often referred to as a disk post), wear and fretting erosion have been observed, particularly in the blade roots carried by the rotor. Composite blades made of stacked or layed-up plies of a reinforced polymeric material; for example an epoxy matrix reinforced with a fiber structure such graphite, glass, boron, etc, as is well known in the art. Examples of such blades are described in U.S. Pat. Nos. 3,752,600; 4,040,770; and 5,292,231. Generally, in such known structures, it has been common practice to dispose metal outserts or metal shells between the blade root and the dovetail slot of the carrying member, in the splayed design conveniently used in such assemblies. The contact between the metal slot of the carrying member and the metal outsert or shell at the juncture between the blade and the slot has resulted in wear and fretting erosion at that interface.
In order to overcome such fretting and subsequent erosion, a composite blade root and a rotor assembly was developed as described in U.S. Pat. No. 5,373,377, entitled "Assembly Of A Composite Blade Root And A Rotor", which is assigned to the General Electric Company, the same assignee as the assignee of this patent and which is hereby incorporated herein by reference. U.S. Pat. No. 5,573,377 discloses an assembly of a plurality of composite blades including blade roots carried by blade root receiving slots in the rotor wherein the slot has a slot wall with a radially outward portion which, when assembled, diverges from a spaced apart juxtaposed blade root pressure face radially outer surface in an amount which is a function of a predetermined amount of centrifugal loading on the blade during operation of the assembly, to allow at least a portion of the radially outer surface of the root pressure face to be in contact with the slot wall radially outward surface during operation. Root outer pads have a plurality of substantially non-metallic, composite plies, rather than metal, bonded with the airfoil structural plies extending into the blade root. A low friction wear coat to help reduce friction induced stresses in the blade root is applied to a root outer pressure face. The wear coat can be applied to and cured on the pressure face and examples of such a coating material include self lubricating films or cloths such as a fabric weave of polytetrafluoroethylene (PTFE) fibers such as Teflon material fibers, glass type fibers, and organic aramid fibers such as Nomex material fibers. Also, a spray of Teflon material or other forms of PTFE material can be used. The low friction coating helps prevent the blades from becoming locked in the rotor slot during deceleration of the rotor during operation. An additional benefit from use of the low friction coating in this combination is the ability of the blade root to slip at a predictable loading condition and provide damping for the blade during resonant crossings and potential blade instabilities, due to the relative motion between the blade base and the rotor slot wall.
A shim disposed between the low friction coat and a slot provides a desired hardness and surface finish to obtain still more improved performance from the low friction wear coat material. The shim is particularly important where the slot wall is a titanium alloy in which desired wear properties are not always achievable. The shim extends the life of the wear coat and prevents wear from occurring to the slot wall is positioned between the wear coat and the slot wall and is both replaceable and removable from the rotor dovetail. The shim can be made of a single material such as steel, titanium or a titanium alloy or it can be a single material having a coating such as copper or a copper alloy on one side.
Fan rotors are balanced in new engines over the engine rotational speed operating range up to redline speeds. Difficulties arise because there is relative fan blade radial and circumferential moment weight changes caused by inconsistent fan blade dovetail seating in the slot associated with break-in of the wear strip. During engine acceptance testing the ran rotor has to be rebalanced after several engine break-in cycles, i.e. ten cycles in one exemplary case, before the proper fan blade dovetail seating is achieved. It is highly desirable to eliminate the need for rebalancing the fan rotor after these break-in cycles.